Method and apparatus for monitoring a process by employing principal component analysis

ABSTRACT

A method and apparatus for monitoring a process by employing principal component analysis are provided. Correlated attributes are measured for the process to be monitored (the production process). Principal component analysis then is performed on the measured correlated attributes so as to generate at least one production principal component; and the at least one production principal component is compared to a principal component associated with a calibration process (a calibration principal component). The calibration principal component is obtained by measuring correlated attributes of a calibration process, and by performing principal component analysis on the measured correlated attributes so as to generate at least one principal component. A principal component having a feature indicative of at least one of a desired process state, process event and chamber state then is identified and is designated as the calibration principal component. Preferably the at least one production principal component is compared to the calibration principal component by computing the inner product of the calibration and production principal components.

FIELD OF THE INVENTION

The present invention relates to techniques for monitoring a process,and more particularly to a method and apparatus for monitoring a processby employing principal component analysis.

BACKGROUND OF THE INVENTION

Within the semiconductor industry, an ever present need exists forimproved process repeatability and control. For example, during theformation of a typical metal-layer-to-metal-layer interconnect, adielectric layer is deposited over a first metal layer, a via hole isetched in the dielectric layer to expose the first metal layer, the viahole is filled with a metal plug and a second metal layer is depositedover the metal plug (e.g., forming an interconnect between the first andthe second metal layers). To ensure the interconnect has low contactresistance, all dielectric material within the via hole must be etchedfrom the top surface of the first metal layer prior to formation of themetal plug thereon; otherwise, residual high-resistivity dielectricmaterial within the via hole significantly degrades the contactresistance of the interconnect. Similar process control is requiredduring the etching of metal layers (e.g., Al, Cu, Pt, etc.), polysiliconlayers and the like.

Conventional monitoring techniques provide only a rough estimate of whena material layer has been completely etched (i.e., endpoint).Accordingly, to accommodate varying thicknesses of material layers(e.g., device variations) or varying etch rates of material layers(e.g., process/process chamber variations), an etch process may becontinued for a time greater than a predicted time for etching thematerial layer (i.e., for an over-etch time). Etching for an over-etchtime ensures that all material to be removed is removed despite devicevariations and process/chamber variations that can vary etch time.

While over-etch times ensure complete etching, over-etching increasesthe time required to process each semiconductor wafer and thus decreaseswafer throughput. Further, the drive for higher performance integratedcircuits requires each generation of semiconductor devices to have finerdimensional tolerances, rendering over-etching increasingly undesirable.The smaller open areas required for reduced dimension device structuresalso reduce the intensity of commonly monitored electromagneticemissions (e.g., reaction product emissions) so as to render monitoringtechniques employing narrow band intensity measurements increasinglydifficult and inaccurate. Accordingly, a need exists for improvedtechniques for monitoring semiconductor manufacturing processes such asetch processes, chamber cleaning processes, deposition processes and thelike.

SUMMARY OF THE INVENTION

The present inventors have discovered that by measuring correlatedattributes of a process (e.g., a plurality of electromagnetic emissions,and/or process temperature, process pressure, RF power, etc.), and byemploying principal component analysis to analyze the correlatedattributes, process state, process event and, if applicable, chamberstate information may be easily and accurately obtained for the process.Exemplary process state information that may be obtained includes RFpower, plasma reaction chemistry, etc.; exemplary process eventinformation that may be obtained includes whether a particular materialhas been etched through or away (i.e., breakthrough), whether a desiredprocess is complete (e.g., etching or deposition), when a wafer isimproperly held (i.e., improper “chucking”), etc.; and, if applicable,exemplary chamber state information that may be obtained includeswhether a chamber contains a fault, whether a chamber's operation issimilar to its previous operation or to another chamber's operation(i.e., chamber matching), etc.

In accordance with the invention, correlated attributes are measured forthe process to be monitored (i.e., the production process), andprincipal component analysis is performed on the measured correlatedattributes so as to generate at least one production principalcomponent. The at least one production principal component then iscompared to a principal component associated with a calibration process(i.e., a calibration principal component).

The calibration principal component is obtained by measuring correlatedattributes of a calibration process (e.g., preferably the same processas the production process, but typically for non-production purposes),and by performing principal component analysis on the measuredcorrelated attributes so as to generate at least one principalcomponent. A principal component having a feature indicative of at leastone of a desired process state, process event and chamber state then isidentified and is designated as the calibration principal component.Preferably the at least one production principal component is comparedto the calibration principal component by computing the inner product ofthe calibration and production principal components. The calibration andproduction principal components also may be compared by employing othertechniques such as the “coherence” function found in the mathematicssoftware package MATLAB™ marketed by Mathworks, Inc. or by computing thescalar magnitude or “norm” of the difference between the calibration andproduction principal components.

By thus comparing calibration and production principal components,process event, process state and chamber state information may beobtained rapidly (e.g., in real time) and with a high degree ofaccuracy. Processes thereby may be monitored and processingparameters/conditions adjusted in real time, over-processing times suchas over-etch times avoided and process yield and throughputsignificantly increased.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit of areference number identifies the drawing in which the reference numberfirst appears.

FIGS. 1A and 1B are a flowchart of an inventive monitoring technique formonitoring a generic process in accordance with the present invention;

FIG. 2 is a schematic diagram of an inventive processing systemcomprising a plasma etching system and an inventive process monitoringsystem coupled thereto in accordance with the present invention;

FIG. 3A is a contour graph of mean-centered optical emissionspectroscopy (OES) information generated the plasma etching of a silicondioxide layer within the processing system of FIG. 2;

FIG. 3B is a cross-sectional diagram of a multilayer semiconductorstructure comprising the silicon dioxide layer etched to obtain the OESinformation of FIG. 3A;

FIG. 3C is a snap-shot of the wavelengths output by a plasma duringetching of the silicon dioxide layer of FIG. 3B;

FIG. 3D is a graph of a first principal component generated duringetching of the silicon dioxide layer of FIG. 3B;

FIGS. 4A and 4B are graphs of the inner product of a calibration and aproduction principal component obtained during etching of the silicondioxide layer of FIG. 3B without and with, respectively, a magneticfield applied during etching;

FIG. 5A is a graph of the inner product of calibration and productionfirst principal components and calibration and production secondprincipal components generated during the etching of a platinummultilayer structure;

FIG. 5B is a cross-sectional diagram of a platinum multilayer structurethat was etched to obtain the graph of FIG. 5A;

FIG. 6A is a graph of the inner product of calibration and productionfirst principal components generated during the etching of a polysiliconmultilayer structure;

FIG. 6B is a cross-sectional diagram of a polysilicon multilayerstructure that was etched to obtain the graph of FIG. 6A;

FIG. 7A is a graph of the inner product of calibration and productionfirst principal components generated during the etching of a BARCmultilayer structure;

FIG. 7B is a cross-sectional diagram of a BARC multilayer structure thatwas etched to obtain the graph of FIG. 7A;

FIG. 8 is a graph of the inner product of calibration and productionfirst principal components generated under processing conditions thatmimic process drift;

FIG. 9 is a schematic diagram of the inventive process monitoring systemof FIG. 2 wherein a dedicated digital signal processor is employed;

FIG. 10 is a schematic diagram of the inventive processing system ofFIG. 2 wherein the process monitoring system is adapted to monitor RFpower, wafer temperature, chamber pressure and throttle valve position;and

FIG. 11 is a top plan view of an automated tool for fabricatingsemiconductor devices that employs the inventive processing system ofFIGS. 2 or 10.

DETAILED DESCRIPTION

As stated, the present inventors have discovered that by measuringcorrelated attributes of a process, and by employing principal componentanalysis to analyze the correlated attributes, process state, processevent and, if applicable, chamber state information may be easily andaccurately obtained for the process. For convenience, the presentinvention is described herein primarily with reference to plasma etchprocesses and plasma-based correlated attributes (e.g., plasmaelectromagnetic emissions, RF power, chamber pressure, throttle valveposition, etc.). However, the invention may be similarly employed tomonitor any other process whether or not a plasma is employed andwhether or not related to semiconductor device processing such asdeposition processes, cleaning processes, chemical-mechanical polishingprocesses, etc. Monitorable correlated attributes for these types ofprocesses include but are not limited to temperature, pressure, weightgain/loss, plasma emissions, RF power, throttle valve position, etc.

FIGS. 1A and 1B are a flowchart of an inventive monitoring technique 100for monitoring a generic process in accordance with the presentinvention. The inventive monitoring technique 100 starts in Step 101.

In Step 102, a process to be monitored (i.e., a production process) isidentified and a calibration process is performed. In most cases, thecalibration process and the production process employ the same processparameters (e.g., identical flow rates, substrate temperatures, chamberpressures, etc.). However, as described below with reference to FIG. 8,to determine the sensitivity of the inventive monitoring technique toprocess drift or to other process variations within the productionprocess, it may be desirable to vary one or more process parameters ofthe calibration process such as process gas flow rates, processtemperature and the like relative to the production process.

During performance of the calibration process, in Step 103, sets ofcorrelated attributes of the calibration process are measured(preferably at a periodic rate) such as a plurality of plasma emissionwavelengths for a plasma process, and/or process temperature, throttlevalve position, process pressure, or any other correlated attributes. Asis known, multiple correlated attributes are required to providesufficient information for principal component analysis.

In Step 104, a time or time period is identified within the collectedcalibration process data that corresponds to a desired process state,process event or chamber state for the calibration process. This time ortime period identification typically is performed following thecalibration process and may therefore be conducted using sophisticated,albeit time consuming, identification techniques not suitable forreal-time use during a process (e.g., during a production process suchas an oxide etch for a contact opening of a semiconductor device). Forexample, if the calibration process is an etch process, the endpoint orbreakthrough time for etching a material layer may be determined byperforming a series of different duration etches under identical processconditions and by examining the cross section (e.g., via scanningelectron or transmission electron microscopy techniques) of the materiallayer for each etch duration to determine the precise endpoint orbreakthrough time for the etching of the material layer. Similarly,process gas flow rates, chamber pressure, process temperature, etc., maybe measured employing sophisticated measurement techniques tocharacterize chamber process state over time or for chamber matchingpurposes.

In Step 105, principal component analysis (PCA) is performed on themeasured correlated attributes for the calibration process collectednear the identified process state, process event or chamber state time.For example, a window of data (e.g., a window comprising data for tendifferent measurement times, or any other window size) comprisingcorrelated attribute data taken at times before, during and/or after theevent can be examined. The correlated attribute data within the windowis used to form a matrix having rows comprising the measured correlatedattribute data and columns comprising the time each attribute set wasmeasured. The data within the matrix may be analyzed as collected butpreferably is mean centered or is mean centered and scaled (as describedbelow). Thereafter a singular value decomposition is performed on thematrix and principal component eigenvectors are generated for themeasured correlated attribute data within the matrix. Typically, two tothree principal components are sufficient to capture 80% of the changesthat occur within the measured correlated attribute data within thematrix.

In Step 106, the generated principal components for the measuredcorrelated attributes of the calibration process are examined forfeatures indicative of the desired process state, process event orchamber state of the calibration process. As described below, typicallyone principal component will contain a sharp feature indicative of thedesired process state, process event or chamber state. In Step 107 theidentified principal component is designated as a “calibration”principal component for the desired process event, process state orchamber state. Once obtained, the calibration principal component may beused to rapidly identify the desired process event, process state orchamber state during the performance of a production process (e.g., inreal time), or thereafter, without requiring the complicated and/or timeconsuming experiments and analysis employed to identify the time withinthe calibration process corresponding to the desired process event,process state or chamber state (described below).

In Step 108, the production process is performed (e.g., typically withthe same process parameters as the calibration process), and, in Step109, correlated attributes for the production process are measured.Preferably during the production process, each time correlatedattributes are measured, the attributes are stored within an evolvingwindow wherein new measured correlated attributes are added to thewindow and old measured correlated attributes are dropped from thewindow over time until all measured correlated attributes pass throughthe window. The evolving window for production process attributes may bethe same size as or a different size from the window used to computecalibration principal components.

In Step 110, each time new measured correlated attributes are added tothe evolving window, principal component analysis is performed on themeasured correlated attribute data therein to generate one or moreprincipal components for the production process (e.g., one or moreproduction principal components). Alternatively, principal componentanalysis may be performed only near the expected time for the desiredprocess state, process event, or chamber state.

In Step 111, at least one production principal component (e.g., the sameorder principal component as the calibration principal component), iscompared to the calibration principal component. The production andcalibration principal components may be compared by any method (e.g.,subtraction, subtraction followed by a norm operation, division, with acoherence-type function, etc.) but preferably are compared by computingthe dot or inner product of the two principal components. Because thetwo principal components have unit length, the inner product of thecalibration and production principal components is approximately +1.0 ifthe calibration and production principal components have approximatelythe same features that change in the same directions, is approximately−1.0 if the calibration and production principal components haveapproximately the same features that change in opposite directions andis approximately zero if the calibration and production principalcomponents do not match. Thus, by taking the inner product of thecalibration and production principal components, the productionprincipal component can be easily compared to the calibration principalcomponent.

In Step 112, a determination is made as to whether the calibration andproduction principal components are approximately the same. If so, inStep 113 a signal is generated indicating that the desired processstate, process event or chamber state has been found during theproduction process, and in Step 116, the inventive monitoring technique100 ends. As described further below, the signal generated indicatingthat the desired process state, process event or chamber state has beenfound may comprise, for example, an indicator that endpoint orbreakthrough has been reached, that process drift has been detected,that a chamber fault has been detected, that chamber matching has beenestablished, etc.

If in Step 112 the calibration and production principal components aredetermined not to match, in Step 114, a determination is made as towhether the production process has ended or has proceeded further thanexpected without detection of the desired process state, process eventor chamber state. If so, in Step 115 a signal (e.g., a warning signal)is generated indicating that the desired process state, process event orchamber state was not found during the production process. Control thenpasses to Step 116 wherein the inventive monitoring technique 100 ends.

If in Step 114 the production process has not ended or has not proceededfurther than expected, control passes to Step 109 where additionalcorrelated attributes are measured for the production process and theadditional measured correlated attributes are added to the evolvingwindow. Principal component analysis then is performed on the datawithin the evolving window (Step 110), a new production principalcomponent is compared to the calibration principal component (Step 111)as previously described. This process repeats until either the desiredprocess state, process event or chamber state is found, or until theproduction process ends or proceeds further than expected. The inventivemonitoring technique 100 now is described with reference to a plasmaprocess.

FIG. 2 is a schematic diagram of a processing system 200 comprising aconventional plasma etching system 202 and an inventive processmonitoring system 204 coupled thereto in accordance with the presentinvention. As used herein, “coupled” means coupled directly orindirectly so as to operate.

The conventional plasma etching system 202 comprises a plasma chamber206 coupled to a plasma etch system controller 208 via a recipe controlport 210 and via a first control bus 212. It will be understood thatwhile a single interface (e.g., the recipe control port 210) is shownbetween the plasma chamber 206 and the plasma etch system controller 208for convenience, in general, the plasma etch system controller 208 mayinterface the various mass flow controllers, RF generators, temperaturecontrollers, etc., associated with the plasma chamber 206 via aplurality of interfaces (not shown).

The plasma chamber 206 comprises a viewport 214 for outputtingelectromagnetic emissions (e.g., primarily optical wavelengths withinthe range from about 180 to 1100 nanometers, generally represented as216 in FIG. 2) from a plasma 218 sustained within the plasma chamber 206(described below). The plasma electromagnetic emissions 216 compriseemissions from a large number of plasma species (e.g., process gasses,reaction products, etc.) and represent one type of correlated attributesthat may be measured for a plasma process. Note that the viewport 214 isshown positioned on the side of the plasma chamber 206, but may bepositioned at any other location (e.g., on the top or bottom of thechamber 206) if desired.

The inventive process monitoring system 204 comprises a spectrometer 220coupled to a processing mechanism (e.g., a processor 222). Thespectrometer 220 is positioned to collect the electromagnetic emissions216 from the plasma 218 and to provide intensity information regarding aplurality of plasma electromagnetic emission wavelengths to theprocessor 222. The spectrometer 220 preferably comprises an Ocean OpticsModel No. S2000 Spectrometer employing a 2048 channel CCD array forproviding intensity information to the processor 222 regarding 2048plasma electromagnetic emission wavelengths spanning a wavelength rangeof about 180 to 850 nanometers. It will be understood that otherspectrometers may be employed and other wavelength ranges may bemonitored. A lens 226 and/or a fiber optic cable 228 preferably aredisposed between the viewport 214 and the spectrometer 220 for improvingcollection of the electromagnetic emissions 216 by the spectrometer 220(e.g., by coupling the electromagnetic emissions 216 into the fiberoptic cable 228 via the lens 226 and by transporting the electromagneticemissions 216 to the spectrometer 220 via the fiber optic cable 228).Other alternative configurations for collecting electromagneticemissions from the plasma 218 may be employed in place of thespectrometer 220 such as a photodiode array wherein each photodiodemonitors a different wavelength or a different wavelength spectrum. Ifdesired, a bundle of fiber optic cables may be coupled to the diodearray wherein each fiber optic cable within the bundle is coupled to aunique photodiode and supplies electromagnetic emissions thereto.Similarly, diffraction gratings, prisms, optical filters (e.g., glassfilters) and other wavelength selective devices may be employed with aplurality of detectors (e.g., photodiodes, photomultipliers, etc.) toprovide information regarding a plurality of electromagnetic emissionwavelengths to the processor 222. The processor 222 is coupled to theplasma etch system controller 208 via a second control bus 230.

In operation, a user 232 (e.g., a person in charge of a waferfabrication process) supplies (via a third control bus 234) the plasmaetch system controller 208 with a set of instructions for generating theplasma 218 within the plasma chamber 206 (i.e., a plasma recipe).Alternatively, a remote computer system for running a fabricationprocess that includes the processing system 200, a manufacturingexecution system or any other fabrication control system may supply theplasma etch system controller 208 with a plasma recipe (e.g., assupplied by the user 232 or as stored within a plasma recipe database).A typical plasma recipe includes processing parameters such as thepressure, temperature, power, gas types, gas flow rates and the likeused to initiate and maintain the plasma 218 within the plasma chamber206 during plasma processing. For example, to perform aluminum etchingwithin the plasma chamber 206, a typical plasma recipe would include atleast the following: a desired chamber pressure, a desired processtemperature, a desired RF power level, a desired wafer bias, desiredprocess gas flow rates (e.g., desired flow rates for process gasses suchas Ar, BCl₃ or Cl₂), etc. Once the plasma etch system controller 208receives a plasma recipe from the user 232, from a remote computersystem, from a manufacturing execution system, etc., the plasma recipeis supplied to the recipe control port 210 via the first control bus212, and the recipe control port 210 (or the plasma etch systemcontroller 208 itself if the recipe control port 210 is not present)establishes and maintains within the plasma chamber 206 the processingparameters specified by the plasma recipe.

During a plasma process within the plasma chamber 206, the plasma 218generates electromagnetic emissions having wavelengths primarily in theoptical spectrum (e.g., from about 180 to 1100 nanometers), althoughboth ultra-violet and infrared wavelengths also may result. A portion ofthese electromagnetic emissions (e.g., the electromagnetic emissions216) travel through the viewport 214 and reach the inventive processmonitoring system 204. Note that while the electromagnetic emissions 216are represented generally by three emission wavelengths in FIG. 2, itwill be understood that the electromagnetic emissions 216 typicallycomprise many more wavelengths.

With reference to FIG. 2, the spectrometer 220 receives theelectromagnetic emissions 216 via the lens 226 and the fiber optic cable228. In response thereto, the spectrometer 220 spatially separates theelectromagnetic emissions 216 based on wavelength (e.g., via a prism ora diffraction grating (not shown)), and generates detection signals(e.g., detection currents) for a plurality of the spatially separatedwavelengths. In the preferred embodiment, an Ocean Optics Model No.S2000 spectrometer is employed for the spectrometer 220 wherein a 600lines/millimeter grating blazed at 400 nanometers spatially separatesplasma emission wavelengths onto a 2048 linear silicon charge-coupleddevice array so as to generate 2048 detection currents or 2048“channels” of detection signal information (i.e., optical emissionspectroscopy (OES) information) for plasma emission wavelengths fromabout 180-850 nanometers. Other wavelength ranges and channel sizes maybe employed if desired, and multiple wavelength regions of the plasmaspectrum may be examined so as to generate multiple calibration andproduction principal components which may be compared in accordance withthe inventive monitoring technique 100.

Once generated, the OES information is digitized (e.g., via ananalog-to-digital converter) and is output to the processor 222 forsubsequent processing (described below). The OES information may beoutput to the processor 222 in analog form if desired. Typically, new2048 channel OES information (e.g., new correlated attribute data) iscollected and supplied to the processor 222 in one second intervals,although other time intervals may be employed.

Because the plasma emission wavelengths collected by the spectrometer220 comprise emissions from a large number of plasma species, thecollected emission wavelengths represent correlated attributes of theplasma process that may be analyzed via principal component analysis.Other suitable correlated attributes of the plasma process include RFpower, wafer temperature, chamber pressure, throttle valve position,process gas flow rates and the like. Thus, in accordance with thepresent invention, correlated attributes (e.g., electromagneticemissions) of the plasma process are measured via the spectrometer 220,and are supplied to the processor 222 in the form of 2048 channels ofOES data. The particular type of processing to be performed by theprocessor 222 preferably is selected by the user 232 (or by a remotecomputer system, by a manufacturing execution system, etc.) via a fourthcontrol bus 236.

FIG. 3A is a contour graph of OES data 300 generated during the plasmaetching of a silicon dioxide layer 302 of a multilayer semiconductorstructure 304 (FIG. 3B). Darker shading in FIG. 3A indicates largermagnitude; and the OES data 300 is mean centered by computing theaverage wavelength intensity between times t₁ and t₂ and by subtractingthe average wavelength intensity from each measured wavelengthintensity. In general, a wavelength intensity occurring at any time t ofinterest may be mean centered, for example, by computing the averagewavelength intensity between times t−10 and t+10 and by subtracting theaverage wavelength intensity from the measured wavelength intensity.

With reference to FIG. 3B, the multilayer semiconductor structure 304comprises the silicon dioxide layer 302 deposited on a silicon wafer 305and having a thickness of about 2000 angstroms, and a photo-resist layer306 deposited on the silicon dioxide layer 302 and having a thickness ofabout 8000 angstroms. The photo-resist layer 306 is patterned to exposeabout 10% of the silicon dioxide layer 302 during etching.

To obtain the OES data 300, the multilayer semiconductor structure 304is placed within the plasma chamber 206 (e.g., a MxP™ chamber with nomagnetic field applied) and the plasma 218 is struck, for example,employing Ar, CHF₃ and CF₄ as is well known in the art. Electromagneticemissions having wavelengths from about 180 to 850 nanometers that passthrough the viewport 214 are collected by the spectrometer 220 and thenon-mean centered OES data 300 is generated by the spectrometer 220. Inthe preferred embodiment, the OES data 300 is generated by taking a“snap-shot” of the wavelengths output by the plasma 218 every second(e.g., 2048 channels of new wavelength data every second) and bydigitizing the data at a rate of about one MHz. Othersnap-shot/digitization rates may be employed. As the OES data 300 iscollected, each wavelength snap-shot preferably is passed to theprocessor 222 in real-time to allow for real time process control of theplasma chamber 206 (described below). The processor 222 mean centers theOES data 300.

FIG. 3C is a snap-shot of the wavelengths output by the plasma 218during etching of the oxide layer 302 (about 60 seconds into the etchingprocess). Conventional monitoring techniques such as endpoint detectionmonitor the change in intensity of individual plasma emissionswavelengths (e.g., the intensity of CF₂ or CO lines) over time. However,as feature sizes continue to shrink for each new semiconductor devicegeneration, less material needs to be etched, fewer reaction productsare generated during etching, less reactive gasses are consumed duringetching, and the changes in individual wavelength intensities that occurduring etching become smaller and more difficult to detect within theoverall plasma emission spectrum. Because principal component analysisexamines multiple correlated attributes (e.g., wavelengths), it is muchless sensitive to a decrease in signal intensity of individual emissionlines that accompanies a decrease in feature size.

With reference to FIG. 3A, etching of the oxide layer 302 begins at timeto and ends somewhere between time t₁ and t₂. As shown in FIG. 3A, themaximum changes in wavelength intensity for the OES data 300 occurbetween time t₁ and t₂, indicative of the etching endpoint for the oxidelayer 302. Specifically, near endpoint, a few wavelengths increase inintensity and a few wavelengths decrease in intensity. However, a sharptransition that identifies the exact location of endpoint is notobservable.

In accordance with the present invention (and the inventive monitoringtechnique 100 of FIGS. 1A and 1B), the plasma process used to generatethe OES data 300 of FIG. 3A is treated as a calibration process; and thepresence and location of the endpoint between times t₁ and t₂ isverified/obtained by independent means (e.g., by a conventional endpointtechnique, by etch studies combined with scanning electron ortransmission electron microscopy, etc.). Principal component analysisthen is performed (as previously described) on a window of OES data nearthe predicted endpoint time (e.g., on a window of about twentywavelength snap-shots encompassing the predicted endpoint time).

FIG. 3D is a graph of the first principal component (PC1) for thecalibration process used to generate FIGS. 3A-3C, computed in thevicinity of the oxide etching endpoint that falls between times t₁ andt₂ (FIG. 3A). The PC1 is defined by “weights” associated with eachwavelength; and the sign and magnitude of each weight associated with awavelength indicates the direction and the magnitude of the changeassociated with the wavelength near endpoint. During subsequentprocessing under identical conditions, the same PC1 component will beobservable near endpoint.

Accordingly, the PC1 of FIG. 3D may serve as a calibration principalcomponent during subsequent “production” processes that “fingerprints”the endpoint event (e.g., the endpoint for the etching of the silicondioxide layer 302 of FIG. 3B).

FIG. 4A is a graph of the inner product of the calibration principalcomponent (e.g., PC1) of FIG. 3D with a production (first) principalcomponent computed during a subsequent etch of the silicon dioxide layer302 of FIG. 3B (employing the same processing conditions used togenerate the OES data 300 of FIG. 3A). No magnetic field was applied. Anevolving window comprising the five most recently obtained wavelengthsnap-shots (from the plasma 218) was employed to generate a newproduction principal component (e.g., a production PC1) every second.Each new production principal component was then compared to thecalibration principal component of FIG. 3D by taking an inner product ofthe two principal components. It will be understood that other windowsizes and other snap-shot rates may be employed.

With reference to FIG. 4A, at time to the plasma 218 is ignited andetching of the silicon dioxide layer 302 begins at time t₁. Etchingcontinues until time t₂. Thereafter, at time t₂, the inner product ofthe calibration and production principal components changes sign from+1.0 to −1.0. This rapid change in the inner product identifies theendpoint for the etching of the oxide layer 302 with a degree of clarityunobservable with conventional endpoint detection techniques. Thepresence of endpoint at time t₂ was verified by other endpoint detectiontechniques.

FIG. 4B is a graph of the inner product of the calibration principalcomponent of FIG. 3D (computed with no magnetic field present duringetching) with a production principal component computed during asubsequent etch of the silicon dioxide layer 302 of FIG. 3B employingthe same processing conditions used to generate the OES data 300 of FIG.3A, but with a 0.25 Hz magnetic field applied within the chamber. As canbe seen in FIG. 4B, even though the calibration principal component wasderived from a process having no magnetic field applied, a sharptransition still exists at time t₂ indicative of the etching endpointfor the silicon dioxide layer 302.

FIG. 5A is a graph of the inner product of a first principal componentfor a calibration process (calibration PC1) with a first principalcomponent for a production process (production PC1) and of the innerproduct of a second principal component for a calibration process(calibration PC2) with a second principal component for a productionprocess (production PC2) generated during the etching of a platinummultilayer structure 501 (FIG. 5B). The platinum multilayer structure501 was etched using a chlorine-based etch chemistry, although any otherknown etch chemistry may be similarly employed.

The platinum multilayer structure 501 comprises a first silicon dioxidelayer 503 deposited on a silicon wafer (not shown) and having athickness of about 2000 angstroms, a titanium nitride layer 505deposited on the first silicon dioxide layer 503 and having a thicknessof about 300 angstroms, a platinum layer 507 deposited on the titaniumnitride layer 505 and having a thickness of about 2000 angstroms, atantalum nitride layer 509 deposited on the platinum layer 507 andhaving a thickness of about 300 angstroms and a second silicon dioxidelayer 511 deposited on the tantalum nitride layer 509 and having athickness of about 6000 angstroms. A portion of the second silicondioxide layer 511 is removed to expose about 60% of the tantalum nitridelayer 509 as shown. Because only about ⅛ of the silicon wafer (notshown) includes multilayer structures such as the multilayer structure501, the net open area to be etched is approximately 7% of the totalwafer area.

The small open area (e.g., about 7%) to be etched is particularlyproblematic for detecting the etching end point of the platinum layer507. Platinum lines overlap intense molecular bands associated with theetch process and limit the use of single line intensity measurements.However, the inventive monitoring technique 100 of FIGS. 1A and 1B caneasily identify the etching endpoint of the platinum layer 507.

To generate a suitable calibration principal component for detectingendpoint for the platinum layer 507 (as well as for titanium nitridelayer 505 and for tantalum nitride layer 509), a series of referenceetch processes were performed on the platinum multilayer structure 501for varying time periods and the platinum multilayer structure 501 wasexamined following each etch process via scanning electron microscopy toidentify the endpoint time for each layer 505-509 (times t₆, t₅ and t₂,respectively, in FIG. 5A). The scanning electron microscopy studiesrevealed that breakthrough of the tantalum nitride layer 509 and etchingof the platinum layer 507 first occur at time t₂, that exposure of thetitanium nitride layer 505 within the open area of the multilayerstructure 501 begins at time t₃, that clearing of the platinum layer 507in dense areas begins at time t₄ and that complete clearing of theplatinum layer 507 occurs at time t₅. Further the titanium nitride layer505 is cleared and the first silicon dioxide layer 503 is exposed attime t₆. Thereafter, to specifically target detection of endpoint forthe platinum layer 507, the calibration PC1 and PC2 were computed neartime t₅ as previously described (e.g., based on plasma emissionwavelengths measured near time t₅). A subsequent “production” etch ofthe platinum multilayer structure 501 was performed under identicalconditions to the reference etch process, and an evolving window wasemployed to generate a new production PC1 and a new production PC2 everysecond.

Each new production PC1 and PC2 was compared to the calibration PC1 andPC2, respectively, by taking an inner product of the first principalcomponents and of the second principal components so as to generate aPC1 inner product curve 513 and a PC2 inner product curve 515,respectively. As shown in FIG. 5A, the etching endpoint for the platinumlayer 507 is clearly identified at time t₅ by the PC1 inner productcurve 513. Further, other etching features of the multilayer structure501 such as plasma ignition at time t₁ and clearing/breakthrough of thetantalum nitride layer 509 at time t₂ are also identifiable. Note thatto more accurately identify the etching endpoint of the titanium nitridelayer 505 or of the tantalum nitride layer 509, calibration principalcomponents may be generated near times t₂ and t₆ and employed within theinventive monitoring technique 100.

FIG. 6A is a graph of the inner product of a calibration PC1 with aproduction PC1 generated during the etching of a polysilicon multilayerstructure 601 (FIG. 6B). The polysilicon multilayer structure 601 wasetched using a bromine-chlorine based etch chemistry, although any otherknown etch chemistry may be similarly employed.

The polysilicon multilayer structure 601 comprises a silicon dioxidelayer 603 deposited on a silicon wafer (not shown) and having athickness of about 1000 angstroms, a polysilicon layer 605 deposited onthe silicon dioxide layer 603 and having a thickness of about 2000angstroms and a photoresist layer 607 deposited on the polysilicon layer605 and having a thickness of about 8000 angstroms. The photoresistlayer 607 is patterned to expose about 25% of the polysilicon layer 605.Based on prior etching experiments and/or knowledge of the inventors, itwas suspected that during etching of the polysilicon multilayerstructure 601, plasma stabilization would occur near time t₁, that CF₄breakthrough would occur near time t₂, that etching of the polysiliconlayer 605 would begin near time t₃, and continue to near time t₄, andthat endpoint for the polysilicon layer 605 would occur near time t₄.

To confirm inventor suspicions, the inventive monitoring technique 100was employed. A calibration PC1 was computed near time t₄ (e.g., basedon plasma emission wavelengths measured near time t₄) and a subsequent,production etch of the polysilicon multilayer structure 601 wasperformed under conditions identical to the calibration etch process. Anevolving window was employed to generate a new production PC1 everysecond, and the calibration and each new production PC1 were compared bytaking an inner product of the principal components so as to generatethe PC1 inner product curve 609 of FIG. 6A. As shown in FIG. 6A, theetching endpoint for the polysilicon layer 605 is clearly identified attime t₄ by the PC1 inner product curve 609. Further, other etchingfeatures of the multilayer structure 601 appear identifiable (e.g.,plasma stabilization at time t₁, CF₄ breakthrough at time t₂, etc.)

FIG. 7A is a graph of the inner product of a calibration PC1 with aproduction PC1 generated during the etching of abottom-anti-reflective-coating (BARC) multilayer structure 701 (FIG.7B). The multilayer structure 701 was etched using a bromine etchchemistry, although any other known etch chemistry may be similarlyemployed.

The BARC multilayer structure 701 comprises a polysilicon layer 703deposited on a silicon wafer (not shown) and having a thickness of about2400 angstroms, a BARC layer 705 deposited on the polysilicon layer 703and having a thickness of about 2000 angstroms, and a photoresist layer707 deposited on the BARC layer 705 and having a thickness of about 8000angstroms. The photoresist layer 707 is patterned to expose about 2% ofthe BARC layer 705.

Because of the very small open area (e.g., 2%) of the BARC multilayerstructure 701, and because photoresist and BARC have a similar materialcomposition, no conventional endpoint techniques can clearly identifythe etching endpoint of the BARC layer 705. However, the inventivemonitoring technique 100 can identify the etching endpoint of the BARClayer 705.

As with the polysilicon multilayer structure 601 of FIG. 6B, based onprior etching experiments and/or knowledge of the inventors, it wassuspected that during etching of the BARC multilayer structure 701,plasma ignition would occur near time t₁, the rim BARC would start toclear near time t₂, the die BARC would start to clear near time t₃ andthe polysilicon layer 703 would be exposed near time t₄ (e.g., the PARClayer 705 would be cleared near time t₄).

To confirm inventor suspicions, the inventive monitoring technique 100was employed. A calibration PC1 was computed near time t₃ (e.g., basedon plasma emission wavelengths measured near time t₃) and a subsequentproduction etch of the multilayer structure 701 was performed underconditions identical to the calibration etch process. An evolving windowwas employed to generate a new production PC1 every second, and thecalibration PC1 and each new production PC1 were compared by taking aninner product of the principal components so as to generate the PC1inner product curve 709 of FIG. 7A. As shown in FIG. 7A, the etchingendpoint for the BARC layer 705 is clearly identified near time by thePC1 inner product curve 709. Further, each etching feature of themultilayer structure 701 also appears identifiable (e.g., plasmaignition at time t₁, clearing of rim BARC at time t₂, and etching of thepolysilicon layer 703 at time t₄).

While the inventive monitoring technique 100 primarily has beendiscussed in terms of endpoint detection with reference to FIGS. 3A-7A,it will be understood that other processing events such as plasmaignition, breakthrough, clearing and the like may be similarlyidentified. Further, the inventive monitoring technique 100 also canprovide information about process state (e.g., RF power, plasma reactionchemistry, etc.) and about a process chamber (e.g., whether a faultexists, whether one chamber matches another chamber, etc.) by providinga “fingerprint” of the plasma process.

With regard to process state information, the shape and the position ofthe various features within the calibration and/or production principalcomponents provide information that may be studied by varying processingparameters or conditions and by examining how the shape and the positionof the features within the principal components change. For example,FIG. 8 is a graph of the inner product of a calibration PC1 with aproduction PC1 under conditions that mimic process drift. A calibrationPC1 was generated by flowing 10 sccms of C₄F₈ during a plasma processwithin an inductively coupled plasma source (IPS) chamber. Thereafter, aproduction process was performed under identical process conditions withthe exception that the flow rate of C₄F₈was increased by 2 sccm every 60seconds. As shown in FIG. 8, the changes in flow rate are easilydiscernible with the inventive monitoring technique 100 (e.g., at 60seconds, 120 seconds, 180 seconds, etc.).

With regard to chamber information, one or more calibration principalcomponent fingerprints of a plasma process taken when the plasma chamber206 is known to be operating properly may serve as a “calibration”fingerprint for the process chamber. Thereafter, the principal componentfingerprints of subsequent process runs may be periodically compared tothe calibration fingerprint for the process. Drift, feature broadening,noise level or other similar changes in the subsequent principalcomponent fingerprints can be quantified to serve as indicators of thehealth of the plasma chamber 206, and can identify chamber faults (e.g.,via unique features attributable to each chamber fault). For example,following a chamber cleaning/maintenance operation, one or moreproduction principal component fingerprints may be measured and comparedto a calibration principal component calibration fingerprint for thechamber to ensure that the chamber is functioning properly following thecleaning/maintenance operation (e.g., as a “chamber qualification”process). The calibration and/or production principal componentfingerprints of two different chambers also may be compared for chambermatching purposes, or to allow one chamber to be adjusted or “equalized”so as to match the principal component fingerprint of another chamber.Any number of production principal components and any principalcomponents (e.g., PC1, PC2, PC3, etc.) for a process may be combined toserve as a calibration fingerprint for the process, if desired.

The inventive monitoring technique 100 may be performed manually (e.g.,by the user 232) or automatically (e.g., by the processor 222) on arun-by-run basis or on a lot-by-lot basis if desired. Preferablycomputation of production principal components is performed as data iscollected during a production process to allow processing parameters tobe adjusted during processing (e.g., in real-time). With reference toFIG. 2, the user 232, a remote computer system for running a fabricationprocess, a manufacturing execution system, etc., may specify whichprocess events (e.g., breakthrough, endpoint, etc.) the processor 222should identify, and whether a warning should be sent to the plasmaetching system 202 via the second control bus 230 in response thereto(e.g., to halt the plasma process within the plasma chamber 206), whatprocess state information is desired (e.g., RF power, plasma reactionchemistry, etc.), whether real-time process control should be employed,what chamber information is desired (e.g., chamber fault information,chamber matching information, etc.) and whether the plasma processwithin the plasma chamber 206 should be halted if a chamber fault isdetected. As stated, only a few plasma emissions wavelengths may bemonitored, if desired.

FIG. 9 is a schematic diagram of the inventive process monitoring system204 of FIG. 2 wherein a dedicated digital signal processor (DSP) 901 isemployed. The DSP 901 preferably is programmed to define the evolvingwindow for production principal component computations and to performprincipal component analysis on the data within the evolving window(described previously) at a significantly higher rate than the processor222. The DSP 901 then supplies the result principal componentinformation to the processor 222 for analysis (e.g., for comparison witha calibration principal component). In this manner, analysis of OES datamay be performed rapidly enough to allow for real-time processingparameter adjustment, if desired. Comparison of production andcalibration principal components also may be performed within the DSP901.

In addition to monitoring plasma emission wavelengths as correlatedattributes of a process, other (or additional) correlated attributes ofa plasma process such as the RF power delivered to a wafer pedestal of aplasma chamber during plasma processing, wafer temperature, chamberpressure, throttle valve position, etc., may be monitored in accordancewith the inventive monitoring technique 100 to obtain process state,process event and chamber information. FIG. 10 is a schematic diagram ofthe processing system 200 wherein the inventive process monitoringsystem 204 is adapted to monitor RF power, wafer temperature, chamberpressure, and throttle valve position during plasma processing ratherthan (or in addition to) plasma emission fluctuations. Specifically,within the inventive process monitoring system 204, the spectrometer 220is no longer shown, and signals representative of the RF power, wafertemperature, chamber pressure and throttle valve position associatedwith the plasma chamber 206 during plasma processing are supplied to theprocessor 222 via a fifth control bus 1000 coupled between the recipecontrol port 210 and the processor 222. If the plasma etch systemcontroller 208 directly interfaces the various mass flow controllers, RFgenerators, temperature controllers, pressure gauges, etc., of theplasma chamber 206 (e.g., without the recipe control port 210),correlated attribute information may be supplied to the processor 222directly from the plasma etch controller 208. It will be understood thatthe spectrometer 220 may be employed to supply OES data to the processor222 along with the other correlated attributes from the recipe controlport 210 or from the plasma etch controller 208 (e.g., RF power, wafertemperature, etc.) if desired.

In general, signals delivered between any components within theprocessing system 200, whether or not delivered over a control bus, maybe delivered in analog or digital form. For example, analog signals maybe digitized via an analog-to-digital converter and transmitted via anRS-232 interface, a parallel interface, etc., if desired.

As with the plasma emission wavelengths, the processor 222 preferablyuses an evolving window to generate a new production principalcomponent, preferably at a period/c rate (e.g., every second), duringthe performance of a production process based on the RF power, wafertemperature, chamber pressure and throttle valve position information.The processor 222 then compares each new production principal componentto a previously generated calibration principal component (as described)so as to obtain process event, process state and chamber information.The DSP 901 of FIG. 9 may be employed with the processor 222 to reduceanalysis time.

FIG. 11 is a top plan view of an automated tool 1100 for fabricatingsemiconductor devices. The tool 1100 comprises a pair of load locks 1102a, 1102 b, and a wafer handler chamber 1104 containing a wafer handler1106. The wafer handler chamber 1104 and the wafer handler 1106 arecoupled to a plurality of processing chambers 1108, 1110. Mostimportantly, the wafer handler chamber 1104 and the wafer handler 1106are coupled to the plasma chamber 206 of the processing system 200 ofFIG. 2 or 10. The plasma chamber 206 has the inventive processmonitoring system 204 coupled thereto (as shown). The entire tool 1100is controlled by a controller 1112 (e.g., a dedicated controller for thetool 1100, a remote computer system for running a fabrication process, amanufacturing execution system, etc.) having a program therein whichcontrols semiconductor substrate transfer among the load locks 1102 a,1102 b and the chambers 1108, 1110 and 206, and which controlsprocessing therein.

The controller 1112 contains a program for controlling the process stateof the plasma chamber 206 in real-time and for monitoring processingevents (e.g., breakthrough, endpoint, etc.) in real-time via theinventive process monitoring system 204 as previously described withreference to FIGS. 1A-10. The inventive process monitoring system 204allows for better control of the process state of the plasma chamber 206and more accurately identifies when processing events occur therein(effectively increasing the throughput of the plasma chamber 206).Accordingly, both the yield and the throughput of the automatedfabrication tool 1100 increases significantly.

In general, the process of measuring correlated attributes for a process(e.g., plasma electromagnetic emissions, RF power, chamber pressure,wafer temperature, throttle valve position, etc.), and the subsequentprincipal component analysis thereof may be performed by a user, by aremote computer system for running a fabrication process, by amanufacturing execution system, etc. As stated, analysis and monitoringpreferably are performed during processing to allow for real-timeprocess control. Preferably a user, a remote computer system for runninga fabrication process, a manufacturing execution system or any othersuitable controller, specifies which process events (e.g., breakthrough,endpoint, etc.) the processor 222 should identify, and whether a warningshould be sent to the plasma etching system 202 in response thereto(e.g., to halt the plasma process within the plasma chamber 206), whatprocess state information is desired (e.g., RF power, plasma reactionchemistry, etc.), whether real-time process control should be employed,what chamber information is desired (e.g., chamber fault information,chamber matching information, etc.) and whether the plasma processwithin the plasma chamber 206 should be halted if a chamber fault isdetected. For example, a library of user selectable functions may beprovided that direct the processor 222 to obtain desired process state,process event and/or chamber information and to act thereuponaccordingly (e.g., to detect the endpoint of an etch process and to haltprocessing thereafter).

To identify processing events such as breakthrough and endpoint, and toobtain process chamber information such as chamber fault information andchamber matching information, a database comprising relevant processevent or process chamber identification information (e.g., calibrationprincipal components that provide endpoint information, breakthroughinformation, chamber matching information, etc.) may be provided withinthe processor 222, within a remote computer system for controlling afabrication process, within a manufacturing execution system, etc. Therelevant information within the database then is accessed by theprocessor 222 and is used to identify process events or to extractchamber information. For example, to detect endpoint or breakthroughduring the etching of a material layer, one or more calibrationprincipal components generated in the vicinity of the breakthrough orendpoint event may be stored within the database. Thereafter, duringprocessing, production principal components may be compared to the oneor more calibration principal components stored within the database. Ifthe production and calibration principal components are within apredetermined range of each other, a signal may be generated to indicatethat either endpoint or breakthrough has been detected. One or morecalibration principal components indicative of endpoint or breakthroughfor each material layer to be etched preferably are stored within thedatabase.

With regard to process chamber information, one or more calibrationprincipal component “fingerprints” of a process taken when the plasmachamber 206 is known to be operating properly may be stored within thedatabase and serve as a “calibration” fingerprint for the processchamber. Thereafter, production principal component fingerprintscomputed during subsequent process runs may be periodically compared tothe calibration fingerprint for the process stored within the database.Drift, feature broadening, noise level or other similar changes in thesubsequent fingerprints can be quantified (e.g., via comparison with thecalibration fingerprint) to serve as indicators of the health of theplasma chamber 206, and to identify chamber faults (e.g., via uniquecalibration or production principal component features attributable toeach chamber fault that are stored within the database). For example,following a chamber cleaning/maintenance operation, a productionprincipal component fingerprint may be measured and compared to apreviously measured calibration principal component calibrationfingerprint for the chamber to ensure that the chamber is functioningproperly following the cleaning/maintenance operation. The calibrationor production principal component fingerprints of two different chambersalso may be compared for chamber matching purposes, or to allow onechamber to be adjusted or “equalized” so as to match the fingerprint ofanother chamber (as previously described). Principal componentfingerprints also may be similarly employed to identify proper waferchucking (e.g., as an improperly chucked wafer will generate uniqueprincipal component features during processing).

The foregoing description discloses only the preferred embodiments ofthe invention, modifications of the above disclosed apparatus and methodwhich fall within the scope of the invention will be readily apparent tothose of ordinary skill in the art. For instance, the monitored plasmaemission wavelength ranges described herein merely are preferred, andother wavelength ranges may be monitored if desired. Productionprincipal components need not be computed using an evolving windowand/or may be computed only in the vicinity of an expected processevent, plasma state or chamber state.

Further, while in FIGS. 2-11 the present invention has been describedwith reference to monitoring the process state of a semiconductor devicefabrication process employing a plasma, it will be understood that ingeneral, the present invention may be used to monitor any process havingmeasurable correlated attributes (e.g., whether or not a plasma isemployed and whether or not related to semiconductor devicefabrication). For example, by monitoring correlated attributes such astemperature, pressure, weight (e.g., via a crystal microbalance),chemiluminescence, etc., of an arbitrary process in accordance with thepresent invention, process state information, process event information,and if applicable, chamber information may be obtained regarding theprocess. As another example, correlated attributes of depositionprocesses (e.g., chemical vapor deposition, plasma enhanced chemicalvapor deposition and high density plasma chemical vapor depositionprocesses for the deposition of silicon nitride, tungsten silicide,polysilicon, low or high K materials, III-V or II-VI semiconductors,fluorinated silicon, triethylphosphate (TEPO) and tetraethylorthosilicate (TEOS) films or any other materials) such as temperature,pressure, weight, plasma emissions, RF power, etc., may be monitored inaccordance with the present invention to obtain process state, processevent and chamber-related information. Such information may be used tomonitor deposition rate, reaction chemistry, RF generator operation,etc., as well as for chamber fault and chamber matching purposes aspreviously described.

Accordingly, while the present invention has been disclosed inconnection with the preferred embodiments thereof, it should beunderstood that other embodiments may fall within the spirit and scopeof the invention, as defined by the following claims.

The invention claimed is:
 1. A method of detecting an endpoint of asemiconductor fabrication process, comprising: (a) performing acalibration process in which a first workpiece is exposed to a plasma;(b) collecting first optical emission spectroscopy (OES) data forelectromagnetic radiation emitted by the plasma during the calibrationprocess; (c) determining a timing of an endpoint that occurred duringthe calibration process; (d) performing principal component analysiswith respect to a window of the first OES data that corresponds to thedetermined endpoint timing to compute a principal component of the firstOES data in the window. (e) after steps (a)-(d), performing a productionprocess in which a second workpiece is exposed to a plasma; (f)collecting second OES data for electromagnetic radiation emitted by theplasma during the production process; (g) for a series of windows of thesecond OES data, performing principal component analysis to compute arespective principal component for each window of the second OES dataand comparing the principal component computed for each window with theprincipal component computed at step (d), wherein the comparing includescalculating inner products of the principal component computed at step(d) with the principal components computed at this step (g); and (h)detecting an endpoint of the production process based on a result ofstep (g).
 2. The method of claim 1, wherein step (h) includes detectinga transition in the calculated inner products.
 3. The method of claim 1,wherein the workpieces are silicon wafers each having a multilayersemiconductor structure.
 4. The method of claim 3, wherein thecalibration process and the production process each include etching alayer of a multilayer semiconductor structure.
 5. The method of claim 4,wherein the etched layer includes silicon dioxide.
 6. The method ofclaim 4, wherein the etched layer includes a metal.
 7. The method ofclaim 4, wherein the etched layer includes polysilicon.
 8. The method ofclaim 4, wherein the etched layer includes abottom-anti-reflective-coating.
 9. The method of claim 1, wherein thefirst and second OES data are collected for electromagnetic emissionshaving wavelengths from about 180 to 850 nanometers.
 10. The method ofclaim 1, wherein the first and second OES data is mean-centered beforeperforming the principal component analyses.
 11. The method of claim 1,wherein step (c) includes microscopic examination of the workpiece. 12.The method of claim 1, wherein steps (f)-(h) are performed concurrentlywith step (e).
 13. The method of claim 1, wherein the series of windowsof the second OES data are generated at intervals of about one second.14. The method of claim 1, wherein the endpoint detected at step (h) iscompletion of an etching process.